1. Field of the Invention
The present invention relates generally to oil field equipment, and more specifically, to a maintenance-free seal for the polish rod of a reciprocating oil well pump.
2. Description of the Related Art
In the oil field industry, the polish rod is the uppermost joint in the string of sucker rods used in a rod pump reciprocal lift system. The purpose of the polish rod is to create an efficient hydraulic seal around the reciprocating rod string. [1] Stuffing boxes, such as those manufactured by Hercules Tool Company and Skinner Brothers Co., Inc., both of Tulsa, Okla., are typically used on polish rods to prevent oil from leaking out of the system. Due to the high pressures of fluid (oil) in the well and the speed at which the rod strings reciprocate, wear and tear on the stuffing boxes is common. Most stuffing boxes are fixed solid to the wellhead tee and do not allow for any flexibility in the pumping unit. The polish rod, however, will flex slightly right and left (laterally) during operation, and this lateral movement causes the packing inside of the stuffing boxes to eventually fail. Thus, either the packing or the stuffing boxes themselves must be periodically replaced. Failure to replace the packing in a timely manner can have catastrophic—and expensive—consequences for the well operator.
Yet another problem with existing stuffing box designs is that when the well is in pump-off mode, it is not moving fluid; therefore, no fluid is present in the stuffing box to lubricate the packing or cool the polish rod. When this happens, the polish rod becomes warm (sometimes hot) and dry. It rubs through the packing rubber, which gets hot and sticky, causing the packing to wear out. Factors affecting how quickly the packing wears out and begins to leak production fluid include: how straight the polish rod is; how long the stroke of the pumping unit is; how fast the unit is pumping; and how straight the pump jack is.
The third problem associated with existing stuffing boxes is that they typically need to be insulated during cold weather to prevent freeze-ups. The stuffing boxes need to be tightened periodically as the packing wears out, but the insulation makes the stuffing box difficult to access, and it also prohibits visual inspection by the pump operator. The present invention makes it more convenient to insulate because no adjustment of the packing is necessary (there is no packing).
The present invention overcomes the disadvantages of prior art stuffing boxes by replacing the stuffing box altogether with a polish rod seal that is specifically designed to accommodate slight lateral movement of the polish rod while still allowing it to accomplish its function. The present invention incorporates a fluid reservoir that prevents the polish rod from over-heating. In addition, the present invention does not utilize nor require packing and is essentially maintenance-free. None of the prior art inventions discussed below solves the same problem in the same manner as the present invention, which is described more fully below in the Detailed Description of Invention section.
U.S. Pat. No. 3,939,910 (Bruce, 1976) provides a stuffing box and blow-out preventing device for polish rods of oil well pumping units. The invention comprises a cylindrical tube with a screw-threaded packing gland in the upper end. An upper guide rubber (i.e., packing) with a plurality of packing rubbers attached above it is engaged by the gland, which applies pressure to seal the polish rod reciprocating within the stuffing box. More specifically, a packing gland nut screw compresses the packing elements relative to the upper wide rubber and polish rod. A blow-out preventor assembly is situated below the upper guide rubber, and O-rings are used to seal the cylindrical rubber portion of the blow-out preventor against the polish rod. An annular chamber is on the outer surface of the cylindrical rubber and is in communication with a source of pressurized fluid. The pressurized fluid is used to apply pressure to the cylindrical rubber portion surrounding the polish rod to shut off the well, and it also acts as an automatic blow-out preventor in the event the polish rod breaks.
U.S. Pat. No. 4,345,766 (Turanyi, 1982) discloses an apparatus for sealing an oil well pump polished rod. The apparatus comprises a tubular housing with two operating tubular portions that are connected to enable limited angular movement between them but also resiliently urged by guide bushings to maintain longitudinal alignment. A plurality of longitudinally spaced packing rings in one of the two housing portions form a seal around the polished rod. Upper and lower rod scrapers protect the seals from coming into contact with any undesired foreign matter that may be carried by the polished rod.
U.S. Pat. No. 4,530,397 (Calhoun, 1985) provides an oil-saving apparatus for use with an oil pump polish rod that attaches to the stuffing box surface. Thus, this invention is intended to be used with a stuffing box, not in lieu of it. The apparatus comprises a housing that attaches to the surface of the stuffing box from which the polish rod emerges. The housing surrounds the polish rod and has an internal collection chamber and an internal packing chamber with auxiliary packing. This invention is intended to collect any fluid that leaks from the stuffing box, but it does not attempt to prevent such leaks.
U.S. Pat. No. 4,613,140 (Knox, 1986) discloses a self-aligning lubricating stuffing box for oil well polish rods. The apparatus comprises a housing situated around the outer periphery of the polish rod with an internal cooling chamber for retaining a fluid reservoir in contact with the polish rod. The apparatus further comprises two bushings (24 and 52 in FIG. 2) that are longitudinally offset from one another and that maintain the polish rod in lateral (axial) alignment. The invention includes a swivel mechanism to compensate for any misalignment between the pump jack and the pumping “T.”
U.S. Pat. No. 5,058,668 (Newton, 1991) provides a rod guide bearing assembly for an oil well pumping apparatus. The invention is a stuffing box assembly comprising a pair of axially spaced bearing assemblies and packing coils positioned between rod guide members at either end of the assembly. Compressive forces are applied to compress the packing coils. Each rod guide member has a liner portion that occupies an annular space between the stuffing box and the polished rod so as to retain axial alignment of the polish rod as it reciprocates. Scraper rings located in internal grooves on the bottom end of the assembly act as the primary seal and also remove scale build-up on the polished rod.
U.S. Pat. No. 5,217,068 (Newton, 1993) discloses an improved stuffing box for a rotary well. The stuffing box comprises upper and lower rod guides with annular packing elements in between them. The lower guide rod is an annular bushing with interior sealing elements at each end of the bushing to form a seal between the bushing and the polished rod. An annular exterior sealing element (O-ring) is situated on the external surface of the bushing (lower rod guide) to form a seal between the bushing and the interior surface of the housing. The invention utilizes a compressive force on the annular packing elements to force them into engagement with the external surface of the polished rod.
U.S. Pat. No. 5,577,737 (Lacy, 1996) provides a method and apparatus for establishing and maintaining a fluid seal around a polishing rod. The invention comprises a fluid responsive gland that forms a seal between the polished rod and the inner surface of the housing. The fluid responsive gland is comprised of packing rings made of supple and rigid materials in alternating configurations. The fluid responsive gland is compressed to maintain engagement with the polished rod. It is comprised of a first self-alignment bushing with an L-shaped cross-section, a second self-alignment bushing with an L-shaped cross-section, and a plurality of V-shaped (chevron) packing rings stacked between the first and second self-alignment bushings.
U.S. Pat. No. 6,412,783 (Finnestad, 2002) discloses a self-aligning stuffing box for pump jacks. The invention comprises a first tubular body having an interior bore with internal stops and a second tubular body having a first end with a concave contact surface. The second tubular body fits telescopically into the first tubular body and is prevented from being withdrawn by virtue of the engagement of the concave contact surface of the second tubular body with the internal stops of the first tubular body. An Annular body with a convex contact surface fits inside of the first tubular body and engages with the concave contact surface of the second tubular body. A third tubular body fits within the first tubular body and exerts a compressive force upon the annular body to ensure that the convex contact surface of the annular body and the concave contact surface of the second tubular body maintain their engagement with one another. This invention has at least five places where the various parts are in threaded engagement with one another. The invention further comprises a tubular packing housing that defines a packing chamber that is adapted to receive packing. The convex surface of the annular body is allowed to move relative to the concave surface of the second tubular body to compensate for misalignment in response to vibration of the polished rod within the axial bore, thereby reducing the rate of wear and tear on the packing and polished rod.
U.S. Pat. No. 7,284,602 (Tessier et al., 2007) provides a self-aligning stuffing box with a spherical joint between the upper and lower portions of the housing that permits the upper and lower portions to move relative to each other in the event that the axis of the polish rod is misaligned. The lower end of the upper housing has a lower concave spherical surface that is upwardly recessed, and the lower housing has an upwardly extending substantially convex spherical surface with a central recess for accepting a downwardly depending central portion of the upper tubular housing and permitting engagement of the concave and convex spherical surfaces. Once engaged, the upper housing is capable of limited universal movement relative to the lower tubular housing. A first sealing means is housed within the downwardly depending central portion of the upper housing and is preferably comprised of a plurality of stacked circumferential chevron seals. A second sealing means is housed in a bore in the upper tubular housing.
U.S. Pat. No. 7,931,078 (Toporowski et al., 2011) discloses a stuffing box apparatus comprising a tubular housing with a bore through it for the polish rod and sealing members situated in the bore in an upper stack and a lower stack. The upper and lower stacks of sealing members are separated by an intermediate bushing. The upper stack is compressed against the intermediate bushing by an upper bushing, and the lower bushing secures the lower stack of sealing members against the bottom of the intermediate bushing. Wiper members form a seal between each bushing and the polish rod.